1. Field of the Invention
The present invention is generally related to broadband communications systems. More particularly, the present invention is related to broadband communications systems that use Data Over Cable Service Interface Specification (DOCSIS) or any of its derivatives.
2. Background Art
In DOCSIS related broadband communications architectures, data is transferred between a central location and many remote subscribers. The central location may be referred to as a Headend for cable systems, a wireless access termination system (WATS) for broadband terrestrial fixed wireless systems, or a satellite gateway for two way satellite systems. Subscriber equipment may be referred to as a cable modem (CM) for cable systems, a wireless modem (WM) for broadband terrestrial fixed wireless systems, or a satellite modem (SM) for two way satellite systems.
In a two-way satellite system, the communication path from the satellite gateway to the SM is called the downstream and the communication path from the SM to the satellite gateway is called the upstream. Satellite modems share a continuous downstream using time domain multiplexing (TDM). Individual satellite modems share a given upstream channel using time domain multiple access (TDMA) techniques.
In standard DOCSIS based systems, upstream frequency control is achieved and maintained as part of the initial and station maintenance DOCSIS mechanisms respectively. As part of the registration process, a cable modem transmits a range request burst during an initial maintenance window. The headend measures the initial frequency offset of the cable modem and sends frequency offset adjustment information back to it as part of a range response message. In DOCSIS based cable applications, the upstream transmit frequency is in the range of 5-65 MHz. Upstream channel bandwidths can be as small as 200 KHz (DOCSIS bandwidth options are 200, 400, 800, 1600 and 3200 KHz). Use of relatively low transmit frequencies insures that the initial frequency offset is small relative to the channel bandwidth. This allows the headend to receive and process the initial range request message. For example, an initial frequency uncertainty of 10 parts per million implies a 650 Hz error on a 65 MHz carrier. Thus, 650 Hz is well within the smallest DOCSIS upstream channel bandwidth of 200 KHz.
In satellite applications, the upstream transmit frequencies are much higher. Typical applications might use Ku band (approximately 14 GHz) or Ka band (approximately 30 GHz). Use of high frequency components introduces higher initial frequency uncertainty. For example an initial frequency uncertainty of 20 parts per million (PPM) implies a 280 KHz error on a 14 GHz carrier and 600 KHz error on a 30 Ghz carrier. These errors may result in a signal being received in a neighboring channel for the smaller DOCSIS upstream channel bandwidths. Such errors are far too large for the use of standard DOCSIS based techniques for initial frequency acquisition and correction at the gateway.
Thus, what is needed is a mechanism whereby initial frequency error of the satellite modem can be controlled and corrected with sufficient accuracy to allow the gateway to receive initial maintenance bursts from the satellite modems with small offsets from the desired frequency.